Thixoforming : Semi-solid Metal Processing

9
Rheocasting of Aluminium Alloys and Thixocasting of Steels
Matthias B€unck, Fabian K€uthe, and Andreas B€uhrig-Polaczek
9.1
Casting of Semi-solid Slurries
The technological and economic potential of innovative materials and their processing
is increasingly in the focus of technology-oriented companies. The demands on
cast parts for the automotive and aerospace industries, but also in wide areas of the
mechanical engineering sector, are still rising. The demand for lighter structures
with higher mechanical properties has to be combined with stringent economic
and ecological aspects. These apparent inconsistent requirements have to be tackled
with the development of innovative manufacturing and material concepts.
Overall, three forming operations have been investigated to produce parts in the
collaborative research centre. Forging processes are performed by using a hydraulic
forging press and the thixocasting is done in a cold chamber high-pressure
die-casting machine.
Thixocasting is a process where an ingot billet is squeezed into a closed die by a
shot piston, comparable to high-pressure die casting. The process is mainly performed
on conventional real-time controlled die-casting machines where the shot
chamber system is adapted to the semi-solid billet insert [1–4].
There are four main aspects for the thixocasting route. First, the velocity of the
slurry is significantly faster than in thixoforging but lower than in conventional highpressure
die casting. A normal piston velocity is about 0.3–1ms 1 for a part thickness
of 5 mm. Together with the flow property of the semi-solid material, this leads to a
laminar die filling. By the closed flow front air enclosed in the die cavity is evenly
conducted through venting channels and the division plane.
Second, the liquid phase can range from 30% to a maximum of 60%. The use of a
faceplate within the gating system reduces the entry and inclusion of damaging
surface oxides from the reheated billet.
A List of Symbols and Abbreviations can be
found at the end of this chapter.
j311